The present invention relates to a polishing apparatus for polishing work pieces, e.g., wafer.
Many types of polishing apparatuses are known.
An ordinary polishing apparatus has a polishing plate, whose upper face is covered with polishing cloth, and a top ring, which has a lower face for holding a work piece and presses a lower face of the work piece onto the polishing cloth. The polishing plate and the top ring are relatively moved so as to polish the lower face of the work piece.
Generally, the work piece is conveyed to a transfer unit by a robot hand, the transfer unit centers the work piece, and the top ring is moved to the transfer unit to receive the work piece.
A conventional polishing apparatus having a transfer unit is disclosed in Japanese Patent Gazette No. 11-347922.
The transfer unit comprises: a pusher stage having a supporting face, on which a work piece, e.g., a semiconductor wafer, will be mounted; an elevating unit for vertically moving the pusher stage; and a guide member, which is located in a periphery of the pusher stage and which has a guide face, which is a female-tapered face.
The semiconductor wafer is conveyed to the pusher stage by the robot hand and mounted onto the supporting face of the pusher stage. Then, the pusher stage is moved downward together with the wafer. The wafer enters the guide member, and an outer circumferential face of the wafer contacts the guide face of the guide member, so that the wafer can be centered. The pusher stage is moved upward, received the centered wafer and presses the centered wafer onto a lower face of the top ring, so that the wafer can be held by the top ring.
After polishing the wafer, the polished wafer is conveyed from the top ring to the transfer unit by the backward process, then the wafer is conveyed from the transfer unit to an external place by the robot hand.
However, in the conventional transfer unit, the work piece is moved from the pusher stage to the female-tapered guide face so as to perform centering of the work piece. When the work piece is transferred to the top ring, the pusher stage is moved upward so as to receive the work piece and press the work piece onto the top ring. Therefore, the centered work piece is vibrated while the pusher stage is moved toward the top ring, thereby the work piece will be easily moved from the center. If the work piece is moved or shifted from the center, the work piece will be damaged or broken.